This invention is in the field of transient devices, and relates generally to passive and active devices designed to programmably transform.
Transient devices have potential for a range of important applications. For example, eco-degradable environmental sensors avoid the need for device collection and bioresorbable medical devices that degrade and are cleared from the body avoid toxicity and inflammation. Strategically, military devices that degrade after a pre-selected time or upon application of a triggered stimulus avoid transferring knowledge or materials to enemies. As a common feature, these devices, and many others, may include printed circuit boards.
Printed circuit boards (PCBs) are essential components of nearly all forms of commercial electronics, where they provide mechanical supports and electrical interconnections for mounted components. Modern PCBs offer multilayer configurations, capable of high frequency signal conduction with minimal transmission loss, in addition to robust and reliable performance in various environmental conditions over extended periods of time. The constituent materials typically include metals such as gold, silver, palladium, tin, and copper, on substrates and interlayer dielectrics of fiber glass or polyimide, for rigid or flexible PCBs, respectively. These materials are designed explicitly for chemical stability and negligible degradation under long-term use.
Increasing demands for the latest technology generations together with an accelerated rate of obsolescence generates expanding volumes of electronic waste (25×106 ton/year), the majority (82%) of which appears in landfills, where the circuits are slow to degrade due to designs that are optimized for robustness and survivability. Release of toxic materials such as lead, mercury, cadmium, polyvinyl chloride, and brominated flame retardants can pollute ground water and soil, with lasting and significant environmental effects that can adversely affect both physiological and psychological wellness. Recovering metals and plastics from PCBs involves chemical separation, burning, and mechanical shredding, each of which involves occupational and environmental hazards.
One potential route to reduce electronic waste streams is to build the electronics using materials that can naturally degrade into biologically and environmentally benign end products on timescales that exceed desired device lifetimes. Recently developed classes of ‘transient’ electronic components, such as transistors, mechanical energy harvesters, electrodes, and primary batteries as well as functional circuits for radio transmission and power harvesting offer these characteristics in systems that involve water soluble active and passive materials. Transient PCBs have not, however, been demonstrated.
Implementation of transient devices is dependent upon design strategies. Design strategies for transient devices may (i) support device fabrication using degradable device component materials and degradable substrates, (ii) provide for accurate control of the useful lifetime of the device, and (iii) utilize materials that are compatible with and perform adequately for a given application within a target environment.
Recently, a number of patents and publications have disclosed devices with transient properties. For example, Kim et al., “Silicon electronics on silk as a path to bioresorbable implantable devices”, Appl. Phys. Lett. 95, 133701 (2009); U.S. Patent Application Publication 2011/0230747; and International Patent Application Publication WO 2008/085904 disclose biodegradable electronic devices that may include a biodegradable semiconducting material and a biodegradable substrate. Bettinger et al., “Organic thin film transistors fabricated on resorbable biomaterial substrates”, Adv. Mater., 22(5), 651-655 (2010); Bettinger et al., “Biomaterial-based organic electronic devices”, Poly. Int. 59(5), 563-576 (2010); and Irimai-Vladu, “Environmentally sustainable organic field effect transistors”, Organic Electronics, 11, 1974-1990 (2010) disclose biodegradable electronic devices that may include a biodegradable organic conducting material and a biodegradable substrate. International Patent Application Publication WO 2008/108838 discloses biodegradable devices for delivering fluids and/or biological material to tissue. U.S. Patent Application Publication 2008/0306359 discloses ingestible devices for diagnostic and therapeutic applications. Kozicki et al., “Programmable metallization cell memory based on Ag—Ge—S and Cu—Ge—S solid electrolytes”, NonVolatile Memory Technology Symposium, 83-89 (2005) discloses memory devices where metal ions within an electrolyte may be reduced or oxidized to form or remove solid metal interconnects.